The invention relates to a differential line driver circuit for driving a line signal output via a line, in particular an xDSL line signal.
Tietze, U.; Schenk, Ch.: Halbleiter-Schaltungstechnik [semiconductor circuitry], 11th edition, Berlin, Springer, 1999, pages 819, 820, 827, ISBN 3-540-64192-0 discloses a voltage-controlled current source for grounded consumers with an adjustable output impedance. The use of two voltage-controlled current sources for floating consumers is likewise known from this document.
U.S. Pat. No. 5,856,758 describes a driver amplifier circuit with a positive feedback. In this case, an impedance is connected between the output terminal and a non-inverting input of an operational amplifier.
FIG. 1 shows a differential line driver circuit according to the prior art. The driver circuit has two inputs E1, E2, to which the input signal Uein, to be amplified is applied. The two driver inputs E1, E2 are respectively connected to the non-inverting inputs of two operational amplifiers OPI, OPII. The two operational amplifiers are voltage feedback operational amplifiers which are supplied with a supply voltage, UV=VDDxe2x88x92VSS. The outputs of the operational amplifiers OPI, OPII are respectively fed back to the inverting inputs (xe2x88x92) via feedback impedances RRI, RRII. The two inverting inputs (xe2x88x92) of the two operational amplifiers OPI, OPII are connected to one another via an adjusting impedance RE for adjusting a voltage gain. The two outputs of the operational amplifiers OPI, OPII are connected to the signal line via line matching impedances RAI, RAII, via the two driver signal outputs A1, A2. The signal line has a load impedance ZLast of, for example, 100 ohms.
The matching impedances RAI, RAII serve for avoiding signal reflections on the signal line L.
It holds for the output impedance of the driver circuit according to the prior art, as is illustrated in FIG. 1, that:
Zaus=RAI+RAII=ZLxe2x80x83xe2x80x83(1)
The two matching impedances RAI, RAII, and the line impedance ZL form a voltage divider, it holding in this case that:
UopAUS=2xc2x7Uausxe2x80x83xe2x80x83(2)
The operational amplifiers OPI, OPII must therefore make two volts of output voltage available for each volt of output voltage UAUS. The output voltage UAUS to be made available at the outputs A1, A2 is prescribed by the technical application and is, for example, 17.6 volts in the case of ADSL driver circuits. The operational amplifiers must therefore make available the voltage UopAUS of 35.2 volts at their output. The supply voltage Uversorgung is:
Uversorgung=VDDxe2x88x92VSSxe2x80x83xe2x80x83(3)
A supply voltage of the two operational amplifiers must be higher in every case than the output voltage UopAUS to be output at its outputs:
Uversorgungxe2x89xa7UopAUS=2xc2x7Uausxe2x80x83xe2x80x83(4)
A supply voltage Uversorgung of 42 volts is typically required for ADSL applications.
The driver circuit according to the prior art illustrated in FIG. 1 therefore has the disadvantage that it requires a very high supply voltage Uversorgung and has a very high power loss, which is caused by the two line matching impedances RAI, RAII.
It is therefore the object of the present invention to create differential line driver circuits which manage with a low supply voltage and have a low power loss.
This object is achieved according to the invention by means of a differential line driver circuit having the features specified in patent claim 1.
The invention creates a differential line driver circuit for driving a line signal output via a signal line, having
a first input terminal for applying a first input signal,
a second input terminal for applying a second input signal,
a first operational amplifier, whose non-inverting input is connected to the first input terminal of the line driver circuit, and whose output is connected to the inverting input via a feedback impedance,
a second operational amplifier, whose non-inverting input is connected to the second input terminal of the line driver circuit, and whose output is connected to the inverting input via a feedback impedance, an adjusting impedance for gain adjustment which is connected between the two inverting inputs of the two operational amplifiers,
a first matching impedance, which is connected between the output of the first operational amplifier and a first output terminal of the line driver circuit,
a second matching impedance, which is connected between the output of the second operational amplifier and a second output terminal of the line driver circuit, the differential line driver circuit additionally having a first positive feedback impedance which is connected between the first output terminal of the line driver circuit and the inverting input of the second operational amplifier, and having a second positive feedback impedance, which is connected between the output of the line driver circuit and the inverting input of the first operational amplifier, the output impedance of the line driver circuit being matched to the impedance of the line.
The output impedance of the differential line driver circuit is preferably fixed by the product of an output impedance synthesis factor and the sum of the impedances of the two matching impedances.
In a preferred embodiment of the line driver circuit according to the invention, the output impedance synthesis factor can be adjusted as a function of the positive feedback impedances and the feedback impedances.
The first positive feedback impedance and the second positive feedback impedance preferably have the same impedances.
The first feedback impedance and the second feedback impedance likewise preferably have the same impedance.
In the case of a first embodiment of the differential line driver circuit, the two operational amplifiers are voltage feedback operational amplifers.
In an alternative embodiment of the differential line driver circuit, the two operational amplifiers are current feedback operational amplifiers.
In a further embodiment of the differential line driver circuit, the adjusting impedance has an infinitely high impedance.
The output impedance synthesis factor is preferably greater than one.
In a particularly preferred embodiment, the output impedance synthesis factor is approximately five.
The differential line driver circuit according to the invention is preferably of fully symmetrical design.
The input impedance of the line driver circuit according to the invention preferably has a high input impedance which corresponds to the high input impedance of an operational amplifier.
The line signals amplified by means of the line driver circuit according to the invention are preferably xDSL signals.
Preferred embodiments of the differential line driver circuit according to the invention are described below with reference to the attached figures in order to explain features essential to the invention.